Gas-phase heteroaromatic substitution. Part 4. Electrophilic attack of t-butyl cation on pyrrole, N-methylpyrrole, furan, and thiophene

Abstract

t-Butyl cation, obtained in the gas state from the γ-radiolysis of neopentane has been allowed to react with pyrrole (1), N-methylpyrrole (2), furan (3), and thiophene (4). Experiments have been carried out in the pressure range 50–760 Torr and in the presence of variable concentrations of a gaseous base (NMe₃). The reactivity of simple heteroaromatic compounds relative to toluene in competition experiments is found to depend markedly upon the composition of the gaseous reaction environment. The apparent k_s/k_T ratios decrease slightly with increasing pressure. At atmospheric pressure, these ratios increase with the NMe₃ concentration levelling off to a value of 1.0 (1), 2.2 (2), 2.6 (3), and 0.6 (4), at the highest base concentration (P_NMe3= 10 Torr). Under the same conditions, predominant β substitution occurs in the pyrroles [β : α : N = 72 : 21 : 7 from (1) and β : α= 53 : 47, from (2)], whereas α-attack is favoured in the case of furan (β : α= 9 : 91) and thiophene (β : α= 20 : 80). The gaseous electrophile appears rather unselective between the n- and the π-type centres of furan (O:ring = 50 : 50) and thiophene (S:ring = 40:60), while it displays no [in the case of (2)] or very limited substitution [7% for (1)] on the heteroatom of the pyrroles. A mechanism based upon reversible addition of the t-butyl cation to the heteroaromatic nucleus, involving preliminary formation of an electrostatic adduct, is used to account for the formation of the t-butylated products and their isomeric composition. It is concluded that the gas-phase t-butyl alkylation of simple five-membered heteroaromatic compounds is regulated by electrostatic interactions established within the encounter pair. A close correspondence exists between this rationalization of the present gas-phase results and recent theoretical and experimental evidence concerning related alkylation processes.